Generally, a pipe joint having internal threads is used for joining together ends of pipes, e.g. water pipes laid under the ground. In those cases, the pipe has external threads formed at the end thereof (in advance), so that the pipe can be coupled by engaging the external threads of the pipe with the internal threads of the pipe joint. However, in this coupling operation, the external threads should necessarily be formed at the end of the pipe beforehand, which requires a lot of time for processing the screw threads and for the connecting operation of the pipes in situ.
In order to solve the above-described problem, the Japanese patent application publication JP10-122460A discloses a pipe joint that comprises a female body, an outer collar, an inner collar, a pressing member, and a coil spring. The coil spring is arranged between the female body and the inner collar. The outer collar has a tapered surface in its inner periphery that increases in diameter toward the female body. The pressing member is constructed such that its outer periphery comes in contact with the tapered surface of the outer collar, whereas the edge formed in its inner surface engages with the outer periphery of the pipe.
In case of this pipe joint, it is not necessary to form external threads at an end of the pipe. The pipe-coupling operation is simplified because it requires only inserting the pipe through the inner collar to the female body. Moreover, when the inner collar is pressed against the female body by the spring outwardly in the axial direction, the pressing member is pressed inwardly in radial direction by the tapered surface of the outer collar. As a result, the edge of the pressing member comes into gripping engagement with the outer periphery of the pipe so as to prevent the pipe from being pulled out of the pipe joint.
In case of the pipe joint having the above-described construction, the inner collar needs to be pressed against the female body outwardly in the axial direction so that the pressing member comes into gripping engagement with the outer periphery of the pipe. Although the coil spring is arranged between the female body and the inner collar, the pressing force of the spring is too weak to make the pressing member strongly engage with the outer periphery of the pipe. If the pipe is made of relatively soft resin, the pressing member may be able to bite into the pipe. However, if the pipe is made of hard resin or metal, it is impossible to make the edge of the pressing member bite into the pipe by means of the pressing force of the coil spring alone. As a result, the pipe joint fails to prevent the pipe from withdrawal. For example, when a large tensile force generated by a high water pressure acts onto the pipe, the pipe may easily be withdrawn from the pipe joint.